1. Field of the Invention
This invention is related to a check valve of the poppet type, and its use in combination with an extruder, for the controlled feeding of a blowing agent into the barrel of an extruder for the extrusion of a polymeric foam resin, e.g. foam polystyrene.
2. Description of the Prior Art
Various apparatus for producing foamed plastic resin articles are known and used in the plastics industry. One such apparatus comprises an extruder including a barrel through which a thermoplastic resin is fed while being melted, an exit head to extrude the molten resin from the barrel to a relatively low pressure area such as the atmosphere, and a fluid injection means to inject a pressurized expandable fluid into the barrel. The fluid, or "blowing agent" as it is called in the industry, is mixed with the molten resin under pressure in the barrel and, when the mixture is extruded out of the barrel into the lower atmospheric pressure environment on the outside of the apparatus, the fluid expands causing the resin to form a foam having the shape and characteristics dictated by the extrusion head and the composition of the resin.
The injection of the blowing agent into the molten resin within the barrel of the apparatus often causes some special problems and resort has been made to the use of one-way check valves in the fluid injection lines to prevent backflow of the resin from the barrel of the extruder into the fluid injection line, the occurrence of which requires subsequent shut-down of the apparatus to remove the blockage. One such type of check valve that has been used in thermoplastic foam extruders comprises a housing having an enlarged internal chamber with fluid feed inlet means at one end portion and fluid outlet means on the other end thereof [see FIG. 2]. Within the enlarged chamber is a spring-loaded ball bearing which acts as the check valve. The arrangement is such that, when there is no fluid pressure in the lines, the spring presses the ball bearing firmly against a restriction constituting the fluid entry port of the valve and thereby prevents any flow either into or out of the valve. When the fluid pressure in the lines is increased to the point where its exerts a force on the ball which is greater then the force exerted by the combined action of the spring and the internal pressure of the apparatus, it will cause the ball to move away from the restriction thereby allowing the fluid to flow around it, through the check valve assembly, and into the barrel of the extrusion apparatus where it is mixed with the resin. In such a ball-bearing type valve, if the force of the fluid pressure within the lines should fall below the combined forces of the spring and the internal pressure of the apparatus acting on the ball, or if there should be a sudden surge of pressure within the apparatus which is greater then the fluid injection pressure, the ball bearing will once again be pressed against its seat and thereby prevent the flow of molten resin through the valve and into the fluid line.
The traditional shutdown procedure for a foam extrusion unit such as described is to sequentially valve off the resin feed line, bleed the molten resin out of unit, and turn off the heaters on the barrel of the extruder and on the fluid (blowing agent) feed line. The blowing agent feed line, which had been kept cool by the fluid flow during the extrusion operation, is warmed by the hot extruder causing the fluid (blowing agent) to expand from the throttling valve through the body of the check valve. The low boiling points of the fluids normally used as blowing agents (e.g. pentane, N.sub.2, Ne, Ar, He) allows them to boil off and completely empty the fluid from the valve into the empty extruder barrel. In that event, if the check valve leaks slightly there will be a small but relatively constant flow of fluid through the valve and into the barrel. If the check valve is tight, the spring pressure causes it to function as a pressure relief valve, in which case most of the fluid feed line would remain full but the check valve would be emptied. In either case, problems may be encountered during start-up and use of the unit which could cause considerable down-time for repair.
To put the extrusion unit back into operation after it has been shut-down and allowed to cool, the heaters are normally turned on before start-up for a sufficient time to allow the extruder barrel, the check valve and other parts of the assembly to reach operating temperature. The extruder drive is then started and resin progresses through the barrel, where it is melted, and into the empty ball-type check valve. The blowing agent feed is turned on and immediately cools and solidifies the molten resin within the body of the ball-type valve, thereby inhibiting passage of the blowing agent through the small metering orifice and into the barrel of the extruder.
In a check valve system of the described ball-bearing type, if the operator in the start-up operation turns the fluid feed (blowing agent) on very slowly, the blowing agent will be warmed before reaching the molten resin present within the valve body and the system may function properly. Similarly, if the valve seat leaks the molten resin in the extruder barrel may never enter the check valve and the system will function. However, any momentary loss of fluid pressure provides a reverse flow through a leaky check valve which, particularly in the case of use of a fluid such as pentane, could create hazardous conditions.
The minimizing or obviation of such undesired problems encountered with check valves of the ball-bearing type are objects of the present invention.